A Digital Television Standard published Sep. 16, 1995 by the Advanced Television Systems Committee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels such as those currently used in over-the-air broadcasting of National Television System Committee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. This is done by positioning the pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal that fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal is offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.6 Hz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690122.4 Hz from the middle frequency of the television channel. The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The number of symbols per horizontal scan line in an NTSC analog TV signal is 684, and 286 is the factor by which horizontal scan line rate in an NTSC analog TV signal is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238*10.sup.6 symbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier. That is, the VSB signal can be limited to a band extending 5.690997 MHz from the lower limit frequency of the television channel.
The ATSC standard for digital HDTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV formats other than HDTV formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM) for terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments. There are 832 symbols per data segment. So, with the symbol rate being 10.76 MHz, each data segment is of 77.3 microseconds duration. Each segment of data begins with a line synchronization code group of four symbols having successive values of +S, -S, -S and +S. The value +S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The initial line of each data field includes a field synchronization code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511-sample pseudo-noise sequence (or "PN-sequence") followed by three 63-sample PN sequences. The middle one of these 63-sample PN sequences is transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one's complementary respective to each other. The other two 63-sample PN sequences and the 511-sample PN sequence are transmitted in accordance with the same logic convention in all data fields.
The remaining lines of each data field contain data that have been Reed-Solomon forward error-correction coded after having been randomized and subjected to diagonal byte interleaving. In over-the-air broadcasting the error-correction coded data are then trellis coded using twelve interleaved trellis codes, each a 2/3 rate punctured trellis code with one uncoded bit. Trellis coding results are parsed into three-bit groups for over-the-air transmission in eight-level one-dimensional-constellation symbol coding, which transmission is made without symbol pre-coding separate from the trellis coding procedure. Trellis coding is not used in cablecasting proposed in the ATSC standard. The error-correction coded data are parsed into four-bit groups for transmission as sixteen-level one-dimensional-constellation symbol coding, which transmissions are made without preceding.
The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed. The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 3-bit symbol coding have normalized values of -7, -5, -3, -1, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized vale of 1.25. The normalized value of +S is +5, and the normalized value of -S is -5.
VSB signals using 8-level symbol coding will be used in over-the-air broadcasting within the United States, and VSB signals using 16-level symbol coding can be used in over-the-air narrowcasting systems or in cable-casting systems. However, certain cable-casting is likely to be done using suppressed-carrier quadrature amplitude modulation (QAM) signals instead, rather than using VSB signals. This has presented television receiver designers with the challenge of designing receivers that are capable of receiving either type of transmission and of automatically selecting suitable receiving apparatus for the type of transmission currently being received.
It is assumed that the data format supplied for symbol encoding is the same in transmitters for the VSB DTV signals and in transmitters for the QAM DTV signals. The VSB DTV signals modulate the amplitude of only one phase of the carrier at symbol rate of 10.76*10.sup.6 symbols per second to provide a real signal unaccompanied by an imaginary signal, which real signal fits within a 6 MHz band because of its VSB nature with carrier near edge of band. Accordingly, the QAM DTV signals, which modulate two orthogonal phases of the carrier to provide a complex signal comprising a real signal and an imaginary signal as components thereof, are designed to have a symbol rate of 5.38*10.sup.6 symbols per second, which complex signal fits within a 6 MHz band because of its QAM nature, with carrier at middle of band.
Processing after symbol decoding is similar in receivers for the VSB DTV signals and in receivers for the QAM DTV signals, assuming the data format supplied for symbol encoding is the same in transmitters for the VSB DTV signals and in transmitters for the QAM DTV signals. The data recovered by symbol decoding are supplied as input signal to a data de-interleaver, and the de-interleaved data are supplied to a Reed-Solomon decoder. Error-corrected data are supplied to a data de-randomizer which regenerates packets of data for a packet decoder. Selected packets are used to reproduce the audio portions of the DTV program, and other selected packets are used to reproduce the video portions of the DTV program.
The design of receivers for both QAM and VSB signals in which QAM/VSB receivers both types of signal are processed through the same interrnediate-frequency amplifiers is described by C. B. Patel et alii in U.S. Pat. No. 5,715,012 issued Feb. 3, 1998, entitled RADIO RECEIVERS FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS and incorporated herein by reference. In a DTV signal receiver as described in U.S. Pat. No. 5,715,012, the presence or absence of VSB pilot carrier in received DTV signal is detected by a pilot-carrier-presence detector in order to generate a QAM/VSB control signal for determining whether the receiver is to be conditioned for operation in a QAM signal reception mode or is to be conditioned for operation in a VSB signal reception mode. During QAM reception there are frequency components 310 kHz from the lower limit frequency of the television channel arising from amplitude modulation of the mid-channel carrier by symbol code description of data, which frequency components fluctuate as to strength. During VSB reception a pilot carrier is transmitted 310 kHz from the lower limit frequency of the television channel, which pilot carrier is supposed to be of constant strength. So long as the pilot carrier received during VSB reception is of constant strength, it can be distinguished from lower-sideband QAM signal modulation of similar frequency. However, during rapidly changing multipath conditions the pilot carrier received during VSB reception may vary in strength sufficiently to make the presence or absence of VSB pilot carrier in received DTV signal difficult or impossible to ascertain.
C. B. Patel et alii describe an alternative method for generating QAM/VSB control signal in U.S. Pat. No. 5,506,636 issued Apr. 9, 1996, entitled HDTV SIGNAL RECEIVER WITH IMAGINARY-SAMPLE-PRESENCE DETECTOR FOR QAM/VSB MODE SELECTION, and incorporated herein by reference. In this alternative method the absence of low-modulating-frequency components in quadrature with the pilot carrier frequency is used to determine that VSB signal is being received. The satisfactory operation of the imaginary-sample-presence detector of U.S. Pat. No. 5,506,636 depends on lack of co-channel interfering signal.
In the QAM/VSB radio receivers described in U.S. Pat. Nos. 5,506,636 and 5,715,012 the final intermediate-frequency signal is digitized, and synchrodyne procedures to obtain baseband samples are carried out in the digital regime. A tuner within the receiver includes elements for selecting one of channels at different locations in a frequency band used for transmitting DTV signals, a succession of mixers for performing a plural conversion of signal received in the selected channel to a final intermediate-frequency (IF) signal, a respective frequency-selective amplifier between each earlier one of the mixers in that succession and each next one of said mixers in that succession, and a respective local oscillator for supplying oscillations to each of the mixers. Each of these local oscillators supplies respective oscillations of substantially the same frequency irrespective of whether the selected DTV signal is a QAM signal or is a VSB signal. The final IF signal is digitized, and thereafter there are differences in signal processing depending on whether the selected DTV signal is a QAM signal or is a VSB signal. These differences are accommodated in digital circuitry including QAM synchrodyning circuitry and VSB synchrodyning circuitry. The QAM synchrodyning circuitry generates real and imaginary sample streams of interleaved QAM symbol code, by synchrodyning the digitized final IF signal to baseband providing it is a QAM signal and otherwise processing the digitized final IF signal as if it were a QAM signal to be synchrodyned to baseband. The VSB synchrodyning circuitry generates a real sample stream of interleaved VSB symbol code, by synchrodyning the digitized final IF signal to baseband providing it is a VSB signal and otherwise processing the digitized final IF signal as if it were a VSB signal to be synchrodyned to baseband. A pilot-carrier-presence detector or an imaginary-sample-presence detector determines whether or not the final IF signal is a VSB signal to generate a control signal, which is in a first condition when the final IF signal apparently is not a VSB signal and is in a second condition when the final IF signal apparently is a VSB signal. Responsive to the control signal being in its first condition, the radio receiver is automatically switched to operate in a QAM signal reception mode; and responsive to the control signal being in its second condition, the radio receiver is automatically switched to operate in a VSB signal reception mode. The satisfactory operation of the imaginary-sample-presence detector of U.S. Pat. No. 5,506,636 depends on achieving synchronized detection of the pilot carrier wave accompanying the VSB DTV signal. Satisfactory operation of the pilot-carrier-presence detector of U.S. Pat. No. 5,715,012 also depends on achieving synchronized detection of the pilot carrier wave accompanying the VSB DTV signal. If there is difficulty with synchronizing with a VSB DTV signal, there is possibility of an undesirable lock-out condition wherein the receiver is placed into a QAM reception mode and control of the frequency and phase of the third local oscillator responsive to baseband VSB demodulation is prevented.
The invention seeks to detect the reception of VSB DTV signal so as to be less susceptible to error caused by rapidly changing multi-path reception, co-channel interference, or failure to completely achieve synchronized detection of the pilot carrier wave. The invention depends on the fact that the data field synchronization segment as symbol coded for VSB transmission comprises pseudo-random noise (PN) sequence information that can be detected using a match filter in a television signal radio receiver capable of receiving VSB DTV signals and at least one other type of television signal. Embodiments of the invention in which the other type of television signal is a QAM DTV signal take advantage of the fact that the data field synchronization segment as symbol coded for QAM transmission does not contain the PN sequence information that can be detected using the same match filter used for detecting the PN sequence information in the VSB transmission.